In general, motorized vehicles, such as the conventional automobile, include a powertrain that is comprised of an engine in power flow communication with a final drive system (e.g., rear differential and wheels) via a multi-speed power transmission. Hybrid type powertrains generally employ an internal combustion engine (ICE) and one or more motor/generator units that operate individually or in concert to propel the vehicle—e.g., power output from the engine and motor/generators are transferred through planetary gearing in the multi-speed transmission to be transmitted to the vehicle's final drive. The primary function of the multi-speed power transmission is to regulate speed and torque to meet operator demands for vehicle speed and acceleration.
To operate properly, the power transmission requires a supply of pressurized fluid, such as conventional transmission oil. The pressurized fluid may be used for such functions as cooling and lubrication. The lubricating and cooling capabilities of transmission oil systems greatly impact the reliability and durability of the transmission. Additionally, multi-speed power transmissions require pressurized fluid for controlled engagement and disengagement, on a desired schedule, of the various torque transmitting mechanisms that operate to establish the speed ratios within the internal gear arrangement.
Transmissions are traditionally supplied with hydraulic fluid by a wet sump (i.e., internal reservoir) oil system, which is separate from the engine's oil system. The fluid is typically stored in a main reservoir or main sump volume where it is introduced to a pickup or inlet tube for communication to the hydraulic pump(s). The pump operates to pressurize the fluid for subsequent communication to the transmission.
It is well known to utilize a fixed displacement (or “PF”, according to industry custom) pump in multi-speed transmissions. A PF pump can generate relatively instantaneous pressure and flow to a hydraulic circuit when the circuit is opened due to the positive displacement characteristic of PF type pumps. In addition to, or in lieu of a PF pump, it is also known to use a variable displacement (or “PV”, according to industry custom) pump to satisfy the hydraulic fluid needs of a multi-speed transmission. The PV pump produces a variable flow on demand. Thus, in standby conditions, PV pump systems do not circulate as much hydraulic fluid.
One premise behind hybrid-type vehicles is that alternative power is available to propel the vehicle, minimizing reliance on the engine for power, thereby increasing fuel economy. Since hybrid-type vehicles can derive their power from sources other than the engine, engines in hybrid-type vehicles typically operate at lower speeds more often than their traditional counterparts, and can be turned off while the vehicle is propelled by the alternative power source(s). For example, electrically-variable transmissions alternatively rely on electric motors housed in the transmission to power the vehicle's driveline.
Engines in hybrid-type vehicles are required to start and stop more often than engines in non-hybrid systems. When the engine in a hybrid-type vehicle is not operating (i.e., in a power-off state), hydraulic pumps which derive their power solely from the engine may become inoperable. As such, many hybrid powertrains include an electrically driven secondary or auxiliary pump that runs independent of the engine—e.g., powered by the vehicle drive lines or a battery, to provide hydraulic pressure during periods when the engine is shutdown.
Packaging space in and around the powertrain in hybrid-type vehicles is normally scarce, often restricting use of a larger auxiliary pump motor. In addition to architectural limitations, installing a larger pump motor is not always possible due to mass, cost, and fuel economy constraints. As such, the motor of a transmission auxiliary pump may be so small that it may not be able to start reliably under certain conditions. However, a transmission auxiliary pump which fails during transition periods (e.g., transition to hybrid “engine-off driving mode”) can potentially result in slip in the transmission launching clutch, and may result in “engine-on” operation only.
Due to size limitations, the auxiliary pump is generally limited in the pressure it can operate against. In most instances, the auxiliary pump operates at significantly less pressure than the engine-driven transmission pump. As such, the auxiliary pump may stall if it is forced to operate at excessive transmission pressures.